Due to thermal drift of the magnet or power supply, or flux creep in the superconducting main magnet coil, a magnetic field from a superconducting circuit may fluctuate or drift.
One approach to minimise deviations and improve the stability of the field generated is to include a field measurement device to measure the magnetic field value and feedback an error signal to the power supply (PSU). A disadvantage of this method is a relatively low level of accuracy and stability since the resolution of the main current PSU, which has an output current of typically 50-1000 A, is of the order of a few PPM and so provides a current output resolution of order 1 mA and so the magnetic field cannot be controlled to an accuracy of better than a few PPM. Another disadvantage is that the reaction time of the power supply is damped due to the stabilising circuitry associated with the DC current transducers.
An alternative approach is to include a passive, coupled, short-circuited inductor in the electrical circuit. The inductor opposes a change of the magnetic field within the magnet. This tends to dampen the change of the magnetic field and lower the magnetic field drift rate. However this method merely slows the rate of magnetic field drift without actually correcting it.
A separately driven ‘shim’ coil controlled by the error signal from the magnetic field measurement device may be provided. The ‘shim’ coil is typically a room temperature coil within the bore of the superconducting magnet, and can provide a small correction signal to control the magnetic field drift. A limitation is that the shim coil has a reduced response due to a low number of turns and therefore does not offer a sufficient amount of correction.